Coronet

A coronet is a small crown consisting of ornaments fixed on a metal ring. By one definition, a coronet differs from other kinds of crowns in that a coronet never has arches, and from a tiara in that a coronet completely encircles the head, while a tiara does not. By a slightly different definition, a crown is worn by an emperor, empress, king or queen; a coronet by a nobleman or lady. See also diadem.

Coronet of an earl (as worn by the 17th Earl of Devon at the Coronation of Elizabeth II and now on display at Powderham Castle)

In other languages, this distinction is not made as usually the same word for crown is used irrespective of rank (Krone in German, kroon in Dutch, krona in Swedish, couronne in French, etc.)

The main use is now actually not on the head (indeed, many people entitled to a coronet never have one made; the same even applies to some monarchs' crowns, as in Belgium) but as a rank symbol in heraldry, adorning a coat of arms.